Controlled chemical release is a desirable characteristic in a number of applications: e.g., prodrug therapy, targeted drug conjugates, or chemical probes of chemical, biochemical, and biological environments. A topic in controlled chemical release that has been the focus of significant research efforts is chemical release of a molecule from a larger entity under different pH conditions. In many applications, it is desirable for a chemical structure to allow little or no chemical release at physiological pH and above, but have the rate of chemical release occur significantly faster at lower pH. Ideal characteristics of such a chemical structure would be that the rate of chemical release could be tuned to desired rates of release at various pH conditions that could be tailored for the structure of the entity being released. The most notable applications requiring such technology is the field of antibody-drug conjugates and drug eluting stents, but it is envisaged that the technology could be applied to prodrug design, intracellular probes, and degradable polymers.
Despite extensive research, most anticancer drugs have nonspecific toxicity and do not explicitly discriminate between healthy and tumor cells. Therefore, they only gain a limited selectivity for malignant cells. Narrow therapeutic windows limit the efficacy of such drugs and result in severe side effects. Due to lack of selectivity, high concentrations of a drug that is required to eradicate the tumor are often not used. In addition, tumors can develop resistance against anticancer drugs after prolonged treatment. Therefore, achieving improved tumor selectivity through targeting of cytotoxic drugs to the cancer cells is needed.
Attaching the cytotoxic drug to a tumor-recognizing ligand (e.g., an antibody, a low molecular weight ligand or cell-specific inhibitor molecules) is considered as one of the promising approaches for tumor specific drug delivery. However, major drawbacks of such attachments are that they can potentially introduce steric hindrances and prevent association of the drug with its molecular target. Therefore, to improve the therapeutic efficacy, the active agent is frequently linked to its targeting ligand through a cleavable spacer that is stable in circulation but readily hydrolyzed or designed to be enzymatically cleaved upon entry into the target cell.
Linkers that are selectively hydrolyzed or decomposed at acidic pH have received considerable attention because the majority of receptor-directed drugs are delivered to endosomal compartments or lysosomes where pH values are thought to be low (Kratz et al., Drug polymer conjugates containing acid-cleavable bonds Crit Rev Ther Drug Carrier Syst 16, 245-288 (1999)). The slightly acidic microenvironment of some tumors (˜pH 6.5) has also been proposed to assist in release of these drugs, especially when the conjugate is expected to be trapped within the tumor for prolonged periods (Gatenby et al., 2006 Acid-mediated tumor invasion: a multidisciplinary study. Cancer Res 66: 5216-5223 (2006)).
Several acid labile linkers have been reported for conjugations of certain classes of small molecules, cytotoxic agents and antibodies. In-depth description of these linkers are available in patent application US 2011/0053878A1, entitled “Acid-labile linkers for drug delivery” by Yang et al; patent application PCT/EP2013/000513, entitled ‘Combinations of albumin-based drug delivery system by Kratz et al. Several other research publications also describe such linkers, for example by Etrych et al. entitled ‘New HPMA copolymers containing doxorubicin bound via pH sensitive linkage: synthesis and preliminary in vitro and in vivo biological properties’, published in the Journal of Controlled Release 73: 89-102 (2002); and by Greenfield et al. entitled ‘Evaluation in vitro of andriamycin immune conjugates synthesized using an acid labile hydrazone linker’, published in Cancer Research Journal 50: 6600-6607 (1997).
Current pH-sensitive linkers suffer from the following shortcomings: 1) slow release of conjugated drugs or molecules at endosomal pH (˜5.5), 2) less than ideal difference in linker stability at pH 5.5 compared to pH 7.4, and 3) limited chemical functional groups allowed for coupling to the linkers thus restricting the scope of cytotoxic drugs to be delivered. The predominate acid-labile linker is the hydrazone, while a possible emerging linker is based on an imidazole scaffold. The common hydrazone linkers are limited to a ketone or aldehyde functionality on the molecular payloads or cytotoxic drugs (e.g., doxorubicin) and the half-lives for the release of drugs range from 2-3 days under acidic conditions (e.g., pH 5.5).
While imidazole linkers allow for tunability and have significantly shorter half-lives for the release of drugs at acidic pH, they also exhibit undesirable release of drug under physiological conditions, which would lead to premature systemic drug release and potentially untoward side effects. The chemical functionalities required on the drugs for coupling to the imidazole linkers appears to be limited to aldehydes or aldehyde-containing spacers.
Although several acid labile linkers are available, there is no such linker which is tunable, can be applied for delivery of a broad range of drugs, and can perform uniformly for different cancer types. Thus, to date the common assumption is that no general linker design exists for all drugs conjugate systems or cancer types. Despite significant progress, each particular cancer types must be examined separately in order to optimize a specific linker based drug delivery system.
Therefore, researchers are looking for pH sensitive linkers which can be enabled for attachment to a wide range of cytotoxic agents, which the stability of the linker can be adjusted for release under broad range of pH conditions, and which can be enabled to rapidly release conjugated therapeutic agents at the target site. These then will provide a means to rapidly synthesize a range of targeted cell-specific drug conjugates that can be released at a controlled rate in a targeted cell-specific environment. Such acid-labile linkers can further be used as part of a delivery mode for compounds other than therapeutic agents, such as, for example, bio-imaging agents. In addition, such linkers can be used for the controlled release of other therapeutic molecules, drugs, or other chemical and biochemical structures from a wide range of matrices and polymers.